THE SACRAL AND COCCYGEAL VERTEBRA 



65 



manently unconnected with the remaining portion of the bone, thus forming a lumbar rib a 

 peculiarity. 



The sacrum, formed by the union of five vertebne, has thirty-five centres of ossification. 



The bodies of the sacral vertebrae have each three ossific centres one for the central part, 

 and one for the epiphyseal plates on its upper and under surface. Occasionally the primary 

 centres for the bodies of the first and second piece of the sacrum are double. The arch of each 

 sacral vertebra is developed from two centres, one for each lamina. These unite with each other 

 behind, and subsequently join the body. 



The lateral masses have six additional centres, two for each of the first three vertebrae. These 

 centres, representing costal- elements, make their appearance above and to the outer side of 

 the anterior sacral foramina (Fig. 34), and are developed into separate segments (Fig. 35); 

 they are subsequently blended with each other, and with the bodies and transverse processes 

 to form the lateral mass. 



Lastly, each lateral surface of the sacrum is developed from two epiphyseal plates (Fig. 36) 

 one for the auricular surface, and one for the remaining part of the thin lateral edge of the bone. 



Additional centres 

 for the first three pieces.* 



At birth. 



Two epiphysial laminse 

 for each lateral surface.* 



At 

 25th year. 



FIG. 34. Development of the sacrum. 



FIG. 35 



FIG. 36 



Centre ft 

 neural 



Centre for 

 al arch. 



Costal 

 element 



Centr, 

 body. 



Period of Development. At about the eighth or ninth week of fetal life ossification of the 

 central part of the bodies of the first three vertebra? commences, and between the fifth and 

 eighth months in the last two. Between the sixth and eighth months ossification of the laminse 

 takes place; and at about the same period the centres for the lateral masses for the first three sacral 

 vertebra? make their appearance. The period at which the arch becomes completed by the junc- 

 tion of the laminse with the bodies in 

 front and with each other behind varies 

 in different segments. The junction 

 between the lamina? and the bodies takes 

 place first in the lower vertebra? as early 

 as the second year, but is not effected in 

 the uppermost until the fifth or sixth year. 

 About the sixteenth year the epiphyses 

 for the upper and under surfaces of the 

 bodies are formed, and between the 

 eighteenth and twentieth years those for 

 each lateral surface of the sacrum make 

 their appearance. The bodies of the 

 sacral vertebrse are, during early life, 



separated from each other by intervertebral disks. At about the eighteenth year the two 

 lowest segments become joined by ossification extending through the disk. This process 

 gradually extends upward until all the segments become united, and the bone is completely 

 formed from the twenty-fifth to the thirtieth year of life. 



The coccyx is developed from four centres, one for each piece. Occasionally one of the first 

 three pieces of this bone is developed from two centres, placed side by side. The ossific nuclei 

 make their appearance in the following order: In the first segment, between the first and fourth 

 years; in the second piece, at from five to ten years; in the third, from ten to fifteen years; in the 

 fourth from fourteen to twenty years. As age advances these various segments become united 

 with each other from below upward, the union between the first and second segments being fre- 

 quently delayed until after the age of twenty-five or thirty. At a late period of life, especially in 

 females, the coccyx often becomes ankylosed to the end of the sacrum. 



5 



Lateral 

 epiphysis. 



FIG. 37. --Base of sacrum at an early stage. 



Lateral 

 epiphysis. 



